The present invention relates to wireless communication, such as provided by systems as specified in 3GPP (Third Generation Partnership Project) Wideband Code Division Multiple Access (WCDMA) release 5, High Speed Downlink Packet Access (HSDPA), but also as provided by other kinds of wireless communications systems. More particularly, the present invention relates to retransmitting via a wireless communication system a portion of a signal when the portion is received with an error; the invention is of use in applications in which both forward error correction and automatic retransmission request are implemented.
To provide for higher data throughput in wireless communication systems, adaptive modulation and coding schemes (MCSs) are used in which both the modulation complexity and (channel) coding complexity are varied in response to changing channel conditions. In some communication systems such as systems implementing HSDPA (high speed downlink packet access), the number of channelization codes (and so the number of channels) can also be varied in response to changing channel conditions. Modulation complexity and channel coding are changed based on rather rapidly changing channel conditions, whereas the number of channels are varied on the basis of a longer-term average, and depending on how much data is to be transmitted. Varying modulation complexity means varying the number of bits that are communicated per symbol (a given modulation complexity provides a set or constellation of symbols, with each symbol used to convey a bit string, the greater the number of symbols in the constellation, the longer the bit string conveyed by each symbol). Varying the coding complexity means, for example, varying the amount of redundancy included in forward error correcting the data to be transmitted. Varying the number of channelization codes means changing the number of channels multiplexed together by use of a code tree (ensuring that all channels remain orthogonal even while the number of channels is varied). The modulation complexity and the number of channelization codes can be adaptively optimized for instance as shown in copending, commonly-owned U.S. Provisional Application Serial No. 60/301,078 filed Jun. 26, 2001. Because the conditions of a wireless communication channel tend to change more often and more substantially than the conditions of a hard-wired channel, errors in communication are more likely. To address the problem of higher error rates (both bit error rates and symbol error-rates), wireless communication systems have implemented various coping mechanisms. One coping mechanism for non-real time data is so-called automatic repeat (retransmission) request (ARQ) protocol, whereby, if a received symbol is determined to have an error, the receiving system automatically requests retransmission of the symbol.
The Problem Addressed by the Invention
Higher-order modulation complexities (higher-order compared to binary systems) include what are generally known as N-QAM (quadrature amplitude modulation) systems (such as e.g. 16-QAM and 64-QAM). N-QAM systems (and other higher-order complexities) convey multiple bits per transmitted symbol. It is inherent in any amplitude modulation system involving more than two symbols (including any N-QAM system for N greater than 2) that the symbol error probabilities are not all the same, i.e. the probability that a receiver will conclude that a symbol was received other than the actually transmitted symbol is different for different symbols. (See e.g. Introduction to Communication Systems, Third Edition, by Ferrel G. Stremler, Addison Wesley Publishing Co., 1990, section 9.5.) Depending on how bits are assigned to the modulation symbols of a coding scheme, the bit error probabilities may vary too, i.e. the probability of a receiver concluding that a 1 was received when a 0 was transmitted (i.e. the bit error probability for a zero) may be different than the probability of a receiver concluding that a 0 was received when a 1 was transmitted.
For instance, in the symbol constellation diagram provided as FIG. 1, showing the constellation currently proposed for high speed downlink packet access (HSDPA), it can be seen that the first two bits are the same for each of the four symbols in any of the four quadrants; in the first quadrant, for example, the first two bits for each symbol are 00. On the other hand, in all of the four quadrants, the last two bits are always 11 in the corner symbols, while they are 00 in the innermost symbols. Consequently, a corner symbol being incorrectly mistaken for an innermost symbol occurs with a different frequency than an innermost symbol being mistaken for a corner symbol. Therefore, a bit having a value of 1 being incorrectly detected as having a value of 0 occurs with a different frequency than a bit having a value 0 being incorrectly detected as having a value of 1. Thus, the bit error probabilities for this constellation are different for 0xe2x80x2s and 1xe2x80x2s.
The notations i1, i2, q1, q2 in FIG. 1 represent the bits in the group constituting a modulation symbol; the bits are in the order i1q1i2q2. A bar under or beside one of the notations (either i1, i2, q1, or q2) indicates where in the constellation diagram the bit indicated by the notation has the value xe2x80x9c1xe2x80x9d (i.e. the bar indicates all or part of the set of constellation points from which the modulation symbol is chosen if a particular bit is 1). For instance, if the bit q1=1, then the symbol must be chosen from the set of points indicated by the bar beside q1 in FIG. 1, and if q1=0, then the symbol must be chosen from the complementary set of points.
A radio receiver has a limited dynamic range. If the modulation symbols have different amplitudes (of the same sign or phase), which indeed is the case in N-QAM modulation, a radio receiver will respond differently to the different symbols on account of their different amplitudes. For instance, the highest amplitude symbols might saturate the receiver, so that the receiver clips those symbols. On the other hand, because the smallest amplitude symbols might be smaller than the smallest quantization level, those symbols might be interpreted by a receiver as having a zero amplitude in the A/D-conversion. Clipping and zeroing are particularly noticeable in a fading channel, where the amplitude of the signal might vary from +10 dB (because of multipath constructive interference) to xe2x88x9240 dB compared to an unfaded signal.
In order to provide both a high data rate (in the downlink) and also reliability, for HSDPA it is proposed that what is termed H-ARQ (for hybrid automatic repeat requests) protocols be employed (at least for data transmission). In H-ARQ, a data packet that has been determined to be in error is retransmitted (when the receiver detects an error in the packet, based for example on some form of simple parity check). The retransmitted packet is combined with the original packet prior to forward error correction (FEC) decoding (in the user terminal receiver, prior to decoding the convolutional or Turbo code), thereby increasing the reliability of the (downlink) transmission.
In a system using either ordinary automatic repeat request (ARQ) or H-ARQ, if the symbols constituting the higher-order modulation system are generated identically in the retransmission and the original transmission, the bit error probabilities in the retransmission are identical to the bit error probabilities in the original transmission. The probability of an error recurring is therefore the same with each retransmission, all other things being the same as when the error first occurred.
Prior Art Solutions
Several H-ARQ techniques have been proposed in HSDPA to improve the likelihood that in case of an error in a packet, a retransmission of the packet will be error-free. The most straightforward is Chase combining, where the same data packet is retransmitted a number of times, and prior to decoding, the repeated transmissions of the coded packet are combined in some fashion. (There are different ways of Chase combining packets, according to the prior art; for example, the packets might be combined on the symbol level, or they might be combined on the soft bit level.)
Another H-ARQ technique is the so-called incremental redundancy (IR) technique, in which the data to be transmitted is encoded with, for instance, a xc2xc FEC code. In the first transmission, only two bits out of the encoded four bits are transmitted per uncoded data bit (i.e. a bit not coded for FEC), and the received signal is decoded as a xc2xd FEC code. If deemed to be in error, the data packet is retransmitted, but with the remaining two encoded bits transmitted per each uncoded bit; the receiver then combines the original and the second transmission, and decodes the received data as a xc2xc FEC code. Since the transmitted encoded bits are different in the different transmissions, the symbol constellations are also different. Hence, in the IR type H-ARQ, the bit error probabilities are different in the retransmissions. IR is believed to minimize the differences in the bit error probabilities. Thus, IR provides that, on average, the bit error probabilities are the same, considering both the original transmission and the retransmission, i.e. when the bit error probabilities are calculated taking both the original transmission and retransmission into account.
There are serious drawbacks in IR compared to Chase combining. IR requires substantially more memory (twice as much), since the number of encoded bits that must be stored in the receiver is four with IR, compared to only two with Chase combining. IR also requires a more complicated FEC decoder, since with IR, the data is first decoded as a xc2xd code (i.e. via a convolutional coder with a xc2xd encoder rate) and then as a xc2xc code.
What is needed is a way to minimize the bit error rate for systems employing higher-order modulation with forward error correction and either ARQ or H-ARQ, without the increased complexity of IR combining.
Accordingly, the present invention provides an apparatus and corresponding method for responding to a repeat request in a wireless communication system in which packets are communicated according to a modulation and coding scheme, each packet encompassing a string of bits, with one or more packets provided in a frame communicated during a transmission time interval, the method including the steps of: accumulating the packets to be conveyed in the next transmission time interval; ordering the bits encompassed by the accumulated packets in a first order so as to provide a first string of bits; providing according to a modulation scheme and a coding scheme the bits so ordered to a process for generating a transmission signal, the modulation scheme providing a constellation indicating a mapping for different groups of a predetermined number of bits to different symbols; transmitting the transmission signal in a next transmission time interval; if a repeat request is received, then ordering the bits encompassed by the accumulated packets in a second order and repeating the method beginning with the step of providing according to a modulation scheme and a coding scheme the bits so ordered to a process for generating a transmission signal; wherein in the second order, the symbols constituting the modulation are generated differently than in the original transmission.
In a further aspect of the invention, the second ordering of the bits is such that the symbols in the modulation constellation are composed differently than in the original transmission. In a still further aspect of the invention, the second ordering of the bits is provided by rearranging the first ordering by transferring a predetermined number of bits of the first string of bits to the end of the first string of bits.
For example, the modulation could be 16 QAM and the predetermined number of bits could be two. In another still further aspect of the invention, the second ordering of the bits is provided by inverting some predetermined number of the last bits in each group of bits that constitutes a symbol. In yet another still further aspect of the invention, the original transmission includes a step of interleaving and the second ordering of the bits is provided using a different interleaving than in the original transmission. In yet even another still further aspect of the invention, the original transmission includes a step of interleaving and the second ordering of the bits is provided using a different data scrambling than in the original transmission.
In another further aspect of the invention, the second ordering of the bits is such that the bits to be retransmitted are mapped to symbols in such a manner that the decoded bit error rate is made smaller.
In yet another further aspect of the invention, in the original transmission, some bits are coded for forward error correction and some are not, and the bits coded for forward error correction are mapped to modulation symbols using a mapping that is not necessarily the same as the mapping used to map to modulation symbols the uncoded bits (which is preferably by Gray encoding), and in the retransmission, the second ordering of the bits is provided using a different symbol mapping, compared to the symbol mapping used in the original transmission, for either the uncoded bits or the bits coded for forward error correction.
In still yet even another further aspect of the invention, in the original transmission, no bits are coded for forward error correction and all bits are mapped to modulation symbols using a first symbol mapping, and in the retransmission, all of the bits are coded for forward error correction, and the second ordering of the bits is provided using a different symbol mapping, compared to the symbol mapping used in the original transmission.
An advantage of the present invention is that it provides (overall) bit error probabilities comparable to those provided with IR H-ARQ, but with less complexity.